Hi Colin - SiTime (www.sitime.com) introduced MEMS-based clock generators (with no external MEMS or quartz) in 2009 and have been shipping in volume since 2010. The three devices (SiT9103, SiT9104, SiT9105) offer single-ended, differential or a combination of single-ended and differential clocks respectively and can support many interface protocols including PCI. The single-ended frequencies go up to 220 MHz and differential frequencies go up to 800 MHz. Differential standards support LVPECL, LVDS, CML and HCSL.
Please see links below for more information.
http://www.sitime.com/products/clock-generators/sit9103
http://www.sitime.com/products/clock-generators/sit9104
http://www.sitime.com/products/clock-generators/sit9105
Thanks
Piyush Sevalia
Exec VP, Marketing, SiTime Corp.

MEMS oscillators are a bizarre market. It is an interesting technology that offers little value (with the exception of shock and vibe) to the customer. While MEMs does offer a solid advantage in microphones and motion-sensing, overall oscillator performance (including the new breed of piezo-actuated devices) only compete with incumbent technologies when David Copperfield math is performed. The price advantage seems completely artificial when one considers the complete picture.

There is the problem of a fractional-N PLL to bring the frequency within tolerance. This adds jitter and power. Quartz still reigns when it comes to high-performance frequency generation. Solutions getting rid of the frac-N PLL will eventually beat out companies like Discera and SiTime. And there are a few companies already out there using CMOS based or other forms of tuned resonators. The quartz solution gets rid of the need for two cascaded PLLs needed when the output frequency is the GHz range. A single PLL coupled to an external quartz xtal or VCXO will have much better performance with less power.

Agreed that the frac-n approach is very problematic. Integer-N approaches rely upon a tunable reference oscillator which is stabilized across temperature and process. That works in the lab at low volume for short periods of time, but it is a million miles from capitalizing upon mature and well understood semiconductor processes. Quartz has nothing to fear, as these integer-n approaches are just science projects.

Have to agree. MEMS timing devices are a solution looking for a problem. MEMS technology is better served for accelerometers, sensors and other miniaturized devices where the competing solutions converting mechanical to electrical signals are not viable or too expensive (in smartphones, tablets, automobiles etc).

The opinions of commentators above seem to indicate MEMS timing devices are currently not a replacements for OCXO' or TCXO's and/or other non-MEMS timing devices that appear to have cost & performance advantages. It would be nice to see poster's like @psevalia contribute a comparative study of this...
MP Divakar